1. Field of the Invention
The present invention relates to a method for determining the weight and thickness of a passivation coating composition on a substrate. Particularly, the invention relates to a passivation coating composition on a metal substrate wherein the metal substrate is a moving coil line.
2. Description of Related Art
A variety of commercial passivation coating compositions are known for preparing metal surfaces to prevent corrosion and improve adhesion of paints or other coatings to the surface. For example, silane pretreatment coatings are used in commercial applications to provide anticorrosion properties to metal surfaces and/or to prepare the metal surfaces prior to painting operations. However, silane pretreatments are extremely difficult to detect by human visual inspection because silane pretreatments are colorless when applied, and at very low coating weights.
Methods for determining the presence of thin films on a substrate are known. Certain fluorescent dyes in metal film coatings have been used to determine whether or not the coating has been applied to the surface. Specifically, the fluorescent brightening agents stilbene and coumarin are added to a chrome-free metal coating and, after covering the metal with the coating, the metal is viewed under ultraviolet (UV) light and the presence of coating is detected by eye. The methods are useful in their ability to determine whether a coating has been applied to a substrate. The methods have been developed which involve quantitative determination of the thickness of the coating, an important quality control matter. Although possible, use of stilbene and coumarin in quantitative determinations of thickness of a coating is not preferred because it has been found that these compounds often do not display the required precision in their use in a calibrated system to determine coating thickness that is required in a commercially feasible coating thickness measurement system. The native fluorescence of the coating can interfere with measurement of fluorescent intensity of the coating.
Methods for determining thickness of transparent oil films on metal surfaces by detection of fluorescent compounds mixed in the oil are also known. However, due to the nature of oil films, these methods are not precise and, therefore, are not suitably reproducible for determination of the thickness of a dryable, dried, curable or cured film-forming coating on a substrate. The layer of oil is typically not maintained on the surface of the substrate if additional layers of a coating are needed on the oil-covered substrate, such as a pre-coating, a primer or a color coat.
It is more difficult to determine the weight and thickness of a passivation coating composition on a moving substrate, for example, a moving coil line, without significantly interrupting the movement of the substrate.
Accordingly, there is a need to be able to quickly detect the presence, weight and thickness of the passivation coating composition as an essential aspect of quality control.